The invention relates to a roller vane pump used for operating an automatic transmission for motor vehicles and in particular for pumping automatic transmission fluid in a continuously variable transmission. The pump is provided with a pump housing, a rotor located in the pump housing and rotatable by means of a drive shaft, a cam ring located around said rotor and roller elements slideably accommodated with some tolerance in slots on the periphery of the rotor. On rotation of the rotor the roller elements interact in a sealing manner with the surface of the cam ring. The cam ring, the rotor, the roller elements and the pump housing define a number of pump chambers, which may arrive in communication with hydraulic channels in the pump housing for allowing flow of fluid to and from the pump chambers. Fluid is communicated between a hydraulic channel and a pump chamber either through one or more suction ports for allowing a predominantly axial flow of fluid to a pump chamber, or through one or more discharge ports for allowing a predominantly axial flow of fluid from a pump chamber.
Such a roller vane pump is known from the European patent 0.555.909 and is in particular adapted for pumping of large volumes of fluid particularly automatic transmission fluid, while maintaining a high pressure in a hydraulically controlled and operated continuously variable transmission for motor vehicles. In a continuously variable transmission of the belt-and-pulley type a large amount of fluid at a high pressure is needed to control the transmission ratio and the belt pinching force, even at a low engine speed. Since the pump is driven by a shaft drivingly connected to the engine shaft, the pump is designed to be able to provide a desired pump yield even at the lowest rotational speed of the engine.
When the pump is operated, the rotor rotates and a low pressure or suction pressure is effected in a pump chamber. Due to the suction pressure fluid is drawn from a hydraulic channel through a suction port or ports into a pump chamber. The flow of the fluid is dependent of said suction pressure and of the surface area of the suction port or ports. Inside a pump chamber, fluid is compressed and subsequently discharged through a discharge port to a hydraulic channel.
Although the known pump functions satisfactory per se, it possesses certain drawbacks. Both the amount of wear of pump parts and the level of noise generated by the pump are not optimal.
The aim of the invention is to optimise the known pump by reducing at least one of wear of pump parts and noise generated by the pump. This aim is, according to the insight underlying the present invention, achieved in providing for a modified rotor and/or a cam ring, the modification being such as to effect an increase of the suction pressure and/or a reduction of the pressure gradient. When a roller element, located in a slot on the periphery of the rotor, has just passed a discharge port, the fluid pressure in a pump chamber in front of that roller element has changed from a high discharge pressure to a much lower suction pressure. The difference between the two pressures is relatively large, as is the pressure gradient associated with said pressure change. Due to said pressure difference and since a roller element is fitted with some tolerance inside a slot, the roller element moves towards the front of the slot as seen in rotational direction of the rotor, where it collides with the rotor generating noise and resulting in wear of the element and of the rotor. Furthermore, inside the known pump the suction pressure becomes low enough for cavitation to occur even at generally occurring pump parameters. Cavitation amounts both to wear of pump parts and to noise generated by the pump, as is commonly known. A pump according to the invention has an improved functionality, since its functional life is prolonged and less noise is generated by the pump during operation. A rotor and a cam ring according to the invention can be adopted both simultaneously and alternatively.
In a first embodiment of the solution according to the invention, the cam ring is provided with a recess constituting a suction port for allowing a predominantly radial flow of fluid to a pump chamber. The recess may be in communication with a hydraulic channel through a state of the art suction port for allowing a predominantly axial flow of fluid. In this case said state of the art suction port is extended radially outward. According to a further development of the solution, the recess is in communication with a hydraulic channel through an additional suction port. Said additional suction port may allow either a predominantly axial or a predominantly radial flow of fluid to the recess. In the latter case fluid is allowed to the recess through the additional suction port from a hydraulic channel located in radial direction outside the cam ring. Since the roller elements are supported by the cam ring, the depth of a recess as seen in axial direction is limited.
A suction port for allowing a predominantly radial flow of fluid to a pump chamber increases the surface area through which fluid is drawn to that pump chamber, thereby increasing the suction pressure. Therefore, the occurrence of cavitation is shifted towards a higher pump yield and/or operating temperature and the pressure gradient during pumping is reduced. Wear of pump parts and noise generated by the pump is reduced.
It is remarked that a fuel pump with a suction port for providing a predominantly radial flow of fluid to a pump chamber, however with a different constitution, is known from the German patent application 3.014.147-A. As opposed to the field of the present invention, a fuel pump is especially adapted for the pumping of fuel. This type of usage requires less flow of a less viscous medium while maintaining a lower pressure. Furthermore, the fuel pump is usually electrically driven, so it can be operated at a constant and freely adjustable rotational speed of the rotor depending on the desired flow. Moreover, the suction port disclosed in DE3.014.147-A is constituted by a hole in the cam ring. This type of port increases the cost of the manufacturing process of the cam ring, because said hole is introduced into the cam ring either by drilling or by a core during the casting or sintering of the cam ring, which increases the complexity and the cost of the process. A suction port according to the invention may be introduced simply by the shape of the cam ring mould.
In an alternative embodiment of the solution according to the invention, a circumference segment of the rotor in between two subsequent slots as seen in axial direction deviates at least partly from a convex shape such, that the surface area of a pump chamber as seen in axial direction is enlarged. In a simple construction of the rotor according to the invention, said circumference segment is an essentially straight. According to the invention the radial dimension of the front of a slot may be less than that of the back of the slot, since a roller element interacts only with the front of a slot when it is near the bottom of said slot. In another embodiment said circumference segment is at least partly concave. In this manner a large additional axial surface area is created, whilst the radial dimension of both the front and the back of a slot may be unaffected. To increase the axial surface area even further, said circumference line straight line is at least partly oriented radially inward in anti-rotational direction.
A rotor according to the invention increases the surface area through which fluid is drawn to a pump chamber, thereby increasing the suction pressure. Therefore, the pressure gradient during pumping is reduced and the occurrence of cavitation is shifted towards a higher pump yield and/or operating temperature. Wear of pump parts and noise generated by the pump is reduced.
In yet another alternative embodiment of the solution according to the invention, the circumference of a slot as seen in axial direction is at least partly curved such, that the curvature of the curved part substantially matches the curvature of the roller element that is located in said slot. In this manner, a surface contact instead of a line contact between roller element and rotor can be effected. If said curved part is part of the front part of the circumference of a slot as seen in rotational direction of the rotor, said rapid movement of a roller element towards said front of the slot is dampened, because fluid is to be squeezed from in between the roller element and the rotor. Therefore, the force of the collision between roller element and rotor is reduced. For optimal results, said curved part substantially starts at the instantaneous radial position of the axial centre line of the roller element, at the instance the roller element starts to interacts with the front of the slot, and continues in a radially inward direction. Said instance occurs immediately after the fluid pressure in the pump chamber in front of the roller element has dropped from the discharge pressure to the suction pressure. Taking into account both functional and manufacturing aspects, the curvature of the curved part preferably extends over a 30 to 90 degree angle. A value over 90 degrees presents manufacturing problems and hinders the radial and or tangential movement of a roller element, whereas a value under 30 degrees results in a negligible damping. According to a further development of this solution, said front part of said circumference consists at least of said curved part and one or more straight parts adjacent to said curved part, to provide support for the roller element and/or to increase the volume of a pump chamber and the surface area through which fluid can be supplied to and discharged from a pump chamber. This effect is enhanced even further if the back part of said circumference extends over a substantially equal radial distance as said front part. Preferably said back part is substantially parallel to a straight part of said front part.
A rotor with slots according to the invention reduces the pressure gradient during pumping and the noise generated by the pump is reduced.
The invention will now be explained in greater detail with reference to the non-restricting examples of embodiment shown in the figures.